MPEG applications - The MPEG - page 232

Figure 7.24: Files are broken into frames or packets for multiplexing with packets from other users. Short packets
minimize the time between the arrival of successive packets. The priority of the multiplexing must favour
isochronous data over asynchronous data.
Figure 7.25 shows the structure of Ethernet which uses a protocol called CSMA/CD (carrier sense multiple access
with collision detect) developed by DEC and Xerox. This is a distributed arbitration network where each node
follows some simple rules. The first of these is not to transmit if an existing bus signal is detected. The second is
not to transmit more than a certain quantity of data before releasing the bus. Devices wanting to use the bus will
see bus signals and so will wait until the present bus transaction finishes. This must happen at some point because
of the frame size limit. When the frame is completed, signalling on the bus should cease. The first device to sense
the bus becoming free and to assert its own signal will prevent any other nodes transmitting according to the first
rule. Where numerous devices are present it is possible to give them a priority structure by providing a delay
between sensing the bus coming free and beginning a transaction. High-priority devices will have a short delay so
they get in first. Lower-priority devices will only be able to start a transaction if the high-priority devices don't need
to transfer.
Figure 7.25: In Ethernet collisions can occur because of the finite speed of the signals. A 'back-off' algorithm
handles collisions, but they do reduce the network throughput.
It might be thought that these rules would be enough and everything would be fine. Unfortunately the finite signal
speed means that there is a flaw in the system. Figure 7.25 shows why. Device A is transmitting and devices B and
C both want to transmit and have equal priority. At the end of A's transaction, devices B and C see the bus become
free at the same instant and start a transaction. With two devices driving the bus, the resultant waveform is
meaningless. This is known as a collision and all nodes must have means to recover from it. First, each node will
read the bus signal at all times. When a node drives the bus, it will also read back the bus signal and compare it
with what was sent. Clearly if the two are the same all is well, but if there is a difference, this must be because a
collision has occurred and two devices are trying to determine the bus voltage at once.
If a collision is detected, both colliding devices will sense the disparity between the transmitted and readback
signals, and both will release the bus to terminate the collision. However, there is no point is adhering to the simple
protocol to reconnect because this will simply result in another collision. Instead each device has a built-in delay